[Bug libstdc++/128429] libstdc++ headers not considered system headers in the build tree

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128429

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The g++ tests could default to using -D_GLIBCXX_SYSHDR so that they see the
user-facing version of the library headers.

That would still mean that libstdc++ tests are compiled with non-system
headers, which is the main benefit.